Hi,
I've tried to reproduce your error, but with no success.
You can look at my source code. There are a few difference between mine and yours, but it works.
The version of the Microsoft.Office.Interop.Word library, that i've test it, is 14.0.0.0, and i've created a document in Office 2010 and save it as a 97-2003 word document.
I taught that if you have in your document only an image, maybe it will cause a null exception, but it dosen't. It gives you an empty string.
object File = @"c:\WordFile.doc";
object nullobject = System.Reflection.Missing.Value;
Microsoft.Office.Interop.Word.Application wordObject = new Microsoft.Office.Interop.Word.Application();
_Document docs = wordObject.Documents.Open(ref File, ref nullobject, ref nullobject, ref nullobject,
ref nullobject, ref nullobject, ref nullobject, ref nullobject,
ref nullobject, ref nullobject, ref nullobject, ref nullobject,
ref nullobject, ref nullobject, ref nullobject, ref nullobject);
docs.ActiveWindow.Selection.WholeStory();
docs.ActiveWindow.Selection.Copy();
IDataObject data = Clipboard.GetDataObject();
string wordString = data.GetData(DataFormats.Text).ToString();
Debug.WriteLine(wordString);
docs.Close(ref nullobject, ref nullobject, ref nullobject);
Hope that this helps you.